Semiconductor wafer with crystal lattice defects, and process for producing this semiconductor wafer

ABSTRACT

A semiconductor wafer has a front side  1 , a back side  2 , a top layer  3 , a bottom layer  4 , an upper inner layer  5  lying below the top layer  3 , a lower inner layer  6  lying above the bottom layer  4 , a central region  7  between the layers  5  and  6  and an uneven distribution of crystal lattice defects. The concentration of the defects exhibits a first maximum (max 1 ) in the central region  7  and a second maximum (max 2 ) in the bottom layer  4.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a semiconductor wafer with anuneven distribution of crystal lattice defects, and to a process forproducing this wafer.

[0003] 2. The Prior Art

[0004] Silicon crystals, in particular for the production ofsemiconductor wafers, are preferably obtained by pulling a seed crystalfrom a silicon melt, which is generally provided inside a quartz glasscrucible. This so-called Czochralski crucible-pulling process isdescribed in detail in, for example, W. Zulehner and D. Huber,Czochralski-Grown Silicon, Crystals 8, Springer Verlag Berlin-Heidelberg1982.

[0005] Due to the reaction of the quartz glass crucible with the moltensilicon during the crucible-pulling process, oxygen is included as thedominant impurity in the growing silicon crystal. The concentration ofoxygen is usually so high that, after the crystal has cooled, it is insupersaturated form. In subsequent heat treatments, the oxygen isdeposited in the form of oxygen precipitates. These precipitations haveboth advantages and disadvantages. The so-called gettering properties ofthe oxygen precipitates are an advantage.

[0006] This is understood to mean that, for example, metallic impuritiesin the semiconductor wafer are bonded to the oxygen precipitates. Thus,they are removed from the layer which is close to the surface and isrelevant for components. A drawback is that oxygen precipitates in thelayer which is close to the surface. This is relevant for components andwill interfere with the function of the components which aremanufactured on the semiconductor wafer. Consequently, it is desired fora precipitate-free zone, PFZ, also known as a denuded zone, DZ, to beformed in the vicinity of the surface. It is also desired for a highconcentration of precipitates to be formed in the interior of thesemiconductor wafer, known as the bulk.

[0007] The prior art, for example in “Oxygen in Silicon” F. Shimura,Semiconductors and Semimaterials Vol. 42, Academic Press, San Diego,1994, has disclosed how the outdiffusion of the oxygen near the surfaceis achieved in a heat treatment at temperatures of preferably over 1100°C. As a result of this outdiffusion, the concentration of oxygen in thelayer close to the surface falls so far that there is no longer anyprecipitation, and consequently a PFZ is generated. This heat treatmentwas in most cases directly integrated into the processes for producingcomponents. In modern processes, however, these high temperatures are nolonger used, and consequently the required outdiffusion is brought aboutby an additional heat-treatment step.

[0008] The oxygen precipitation, in particular in crucible-pulledsemiconductor material, takes place substantially in two steps:

[0009] 1) formation of nucleation centers for oxygen precipitates,so-called nuclei;

[0010] 2) growth of these centers to form detectable oxygenprecipitates.

[0011] During subsequent heat treatments, the size of these nuclei canbe modified in such a way that those which have a larger radius than theso-called “critical radius” grow into oxygen precipitates. On the otherhand, nuclei with a smaller radius break down (are dissolved). Thegrowth of nuclei with a radius >r_(c) takes place at elevatedtemperature and is substantially limited by the diffusion of oxygen. Agenerally accepted model (cf., for example, Vanhellemont et al., J.Appl. Phys. 62, p. 3960, 1987) describes the critical radius r_(c) as afunction of the temperature, the oxygen supersaturation and theconcentration of vacancies. Concentration is understood to meanparticles per unit volume.

[0012] A high oxygen concentration and/or a high vacancy supersaturationsimplifies or accelerates the precipitation of oxygen and leads to ahigher concentration of precipitates. Furthermore, the concentration orsize of the precipitates, in particular in semiconductor wafers, dependson heating and cooling rates during thermal furnace processes, inparticular during the so-called RTA (rapid thermal annealing) processes.During these heat treatments, semiconductor wafers are heated totemperatures of up to 1300° C. within a few seconds and are then cooledat rates of up to 300° C./sec.

[0013] The oxygen concentration, the vacancy concentration, theinterstitial concentration, the dopant concentration and theconcentration of existing precipitation nuclei, such as for examplecarbon atoms, also influence the precipitation of oxygen.

[0014] WO 98/38675 has disclosed a semiconductor wafer with an unevendistribution of crystal lattice vacancies, which is obtained by means ofa heat treatment. The maximum level of this vacancy profile generated inthis way is situated in the bulk of the semiconductor wafer, and theprofile decreases considerably toward the surfaces. During subsequentheat-treatment processes, in particular at 800° C. for 3 h and 1000° C.for 16 h, the oxygen precipitation follows this profile. The result is aPFZ without prior outdiffusion of the oxygen and oxygen precipitates inthe bulk of the semiconductor wafer.

[0015] According to WO 98/38675, the concentration of oxygenprecipitates is set by means of the concentration of vacancies, and thedepth of the precipitates is set by means of the cooling rate followingthe heat treatment. A drawback of this semiconductor wafer is that thegetter centers are limited to the bulk. Furthermore, very high BMD (bulkmicro defect) concentrations lead to high leakage currents fromintegrated circuits when these circuits are located close to the layerrelevant for the components. These leakage currents can be minimized ifregions with very high BMD concentrations are produced as far away fromthe components as possible. Furthermore, it has been found that, inparticular for applications in micromechanics, high BMD concentrationsin the middle of the semiconductor wafer have an adverse effect on theselective etching behavior. This is because a high variation in etchingremoval rates is observed in the presence of the precipitates.Consequently, it is desired to limit the oxygen precipitates as far aspossible to defined layers. It is also desired to keep the back-sidepart of the semiconductor wafer precipitate-free, for example for theproduction of micromechanical structures.

SUMMARY OF THE INVENTION

[0016] It is therefore an object of the present invention to provide asemiconductor wafer, and a process for producing the wafer, which servesas a basis for a semiconductor wafer with an improved internal getteringaction.

[0017] This above object is achieved according to the invention by meansof a semiconductor wafer having a front side 1, a back side 2, a toplayer 3, a bottom layer 4, an upper inner layer 5 lying below the toplayer 3, a lower inner layer 6 lying above the bottom layer 4, a centralregion 7 between the layers 5 and 6 and an uneven distribution ofcrystal lattice defects. The concentration of the defects exhibits afirst maximum (max₁) in the central region 7 and a second maximum (max₂)in the bottom layer 4.

[0018] The defects are preferably vacancies which are converted intonucleation centers for oxygen precipitates during subsequent heattreatment processes, preferably at temperatures of from 300° C. to 800°C. According to the invention, the nucleation centers follow the profileof the vacancies. Preferably, the concentration of the defects increasesfrom the front side 1 of the semiconductor wafer toward the centralregion 7, up to the first maximum (max₁), and toward the bottom layer 4,up to the second maximum (max₂).

[0019] Accordingly, the above object is also achieved by means of asemiconductor wafer with an uneven distribution of nucleation centersfor oxygen precipitates.

[0020] In particular, the object is also achieved by means of asemiconductor wafer having a front side 1, a back side 2, a top layer 3,a bottom layer 4, an upper inner layer 5 lying below the top layer 3, alower inner layer 6 lying above the bottom layer 4, a central region 7between the layers 5 and 6 and an uneven distribution of nucleationcenters for oxygen precipitates. The concentration of the nucleationcenters exhibits a first maximum (max₁) in the central region 7 and asecond maximum (max₂) in the bottom layer 4, and the concentration ofthe nucleation centers on the front side 1 and in the top layer 3 is solow that, in a subsequent heat treatment without outdiffusion of oxygen,a precipitate-free layer with a thickness of from 1 to 100 μm is formedon the front side 1.

[0021] Surprisingly, it has been found that the nucleation centers arenot mobile point defects, but rather immobile deposits. According to theinvention, therefore, the oxygen precipitation exactly follows thisprofile during subsequent heat treatments, for example during a heattreatment for 3 h at 780° C. and for 16 h at 1000° C.

[0022] Due to the variation in concentration of the nucleation centersat increasing distance from the surface of the semiconductor wafer, thesubsequent heat treatment processes thus result in a depth-dependentvariation in the concentration of the oxygen precipitates.

[0023] Accordingly, a semiconductor wafer whose concentration ofnucleation centers is very low on the front side 1 and in the top layer3, increases toward the central region, to a first maximum (max₁), andthen rises again toward the back side 2, in order to reach a secondmaximum (max₂) in the bottom layer 4, is preferred.

[0024] However, the object is also achieved by means of a semiconductorwafer having a front side 1, a back side 2, a top layer 3, a bottomlayer 4, an upper inner layer 5 lying below the top layer 3, a lowerinner layer 6 lying above the bottom layer 4, a central region 7 betweenthe layers 5 and 6 and an uneven distribution of crystal latticedefects, wherein the concentration of the defects exhibits a maximum(max₁) in the upper inner layer 5.

[0025] The defects are preferably vacancies which are converted intonucleation centers for oxygen precipitates during subsequent heattreatment processes, preferably at temperatures of from 300° C. to 800°C. According to the invention, the nucleation centers follow the profileof the vacancies.

[0026] Accordingly, the object is also achieved by means of asemiconductor wafer with an uneven distribution of nucleation centersfor oxygen precipitates.

[0027] In particular, the object is also achieved by means of asemiconductor wafer having a front side 1, a back side 2, a top layer 3,a bottom layer 4, an upper inner layer 5 lying below the top layer 3, alower inner layer 6 lying above the bottom layer 4, a central region 7between the layers 5 and 6 and an uneven distribution of nucleationcenters for oxygen precipitates. The concentration of the nucleationcenters exhibits a maximum (max₁) in the upper inner layer 5. Theconcentration of the nucleation centers on the front side 1, the backside 2, in the top layer 3, the bottom layer 4 and the lower inner layer6 is so low that, in a subsequent heat treatment without outdiffusion ofoxygen, precipitate-free layers with a thickness of from 1 to 100 μm areformed on the front side 1 and the back side 2.

[0028] According to the invention, therefore, the oxygen precipitationexactly follows this profile during subsequent heat treatments, forexample during a heat treatment for 3 h at 780° C. and for 16 h at 1000°C.

[0029] The oxygen precipitates in particular in the upper inner layer 5have proven advantageous, since the back-side region of precipitatesremains clear and therefore offers ideal conditions for, for example,applications in micromechanics. Nevertheless, the precipitates formed inthe layer 5 and in the central region 7 produce a good gettering action.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Other objects and features of the present invention will becomeapparent from the following detailed description considered inconnection with the accompanying drawings which disclose embodiments ofthe present invention. It should be understood, however, that thedrawings are designed for the purpose of illustration only and not as adefinition of the limits of the invention.

[0031] In the drawings, wherein similar reference characters denotesimilar elements throughout the several views:

[0032]FIG. 1 shows one embodiment according to the invention in whichthe concentration of defects has a first maximum (max₁) and a secondmaximum (max₂);

[0033]FIG. 2 shows another embodiment according to the invention inwhich the concentration of defects has a maximum in the upper innerlayer; and

[0034]FIG. 3 shows a cross sectional view of a semiconductor wafer ofthe invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0035] According to the invention, the basis for a semiconductor waferwith improved internal gettering action is provided, in a semiconductorwafer, by a vacancy profile which has two maximums (illustrated by wayof example in FIG. 1) or one maximum which is located between the frontside 1 and the central region 7 of the semiconductor wafer (illustratedby way of example in FIG. 2). The nucleation centers exactly followthese profiles during subsequent heat treatment processes.

[0036]FIG. 3 shows the layers defined above as well as the front side 1,the back side 2 and the central region 7 between the layers 5 and 6 of asemiconductor wafer 10. The top layer 3 is preferably between 0 and 100μm thick, the upper inner layer 5 is preferably between 15 and 330 μmthick, the lower inner layer 6 is preferably between 100 and 300 μmthick, and the bottom layer 4 is preferably between 0 and 250 μm thick.

[0037] The concentration of the nucleation centers on the front side ispreferably less than 10⁴ cm⁻³, and ideally the concentration approacheszero. At increasing distance from the front side 1 toward the centralregion 7, the concentration increases, then remains constant in thatregions before rising again toward the back side 2, without outdiffusionof oxygen taking place (FIG. 1). The profile shown in FIG. 2 is likewiseconverted into a corresponding profile of nucleation centers.

[0038] The figures indicate, by way of example, BMD concentrationswithout units. BMDs are to be understood as meaning, in general terms,crystal defects, such as for example vacancies or nucleation centers.

[0039] According to the invention, a precipitate-free layer with athickness of preferably from 1 to 100 μm is obtained on that side of thesemiconductor wafer which is active for the components, without complexoxygen outdiffusion steps having to be carried out.

[0040] Preferably, oxygen precipitates are formed in the upper innerlayer 5, in the lower inner layer 6 and in the bottom layer 4 during thecomponent production, ensuring an improved internal gettering action.

[0041] Furthermore, this special nucleation-center profile shown in FIG.1 brings about improved gettering properties on the back side of thesemiconductor wafer, since that is where the highest concentration ofprecipitates is to be found.

[0042] Preferably, oxygen precipitates form only in the upper innerlayer 5 lying below the top layer 3. This ensures that the region withthe highest precipitate concentration is situated furthest away from thelayer which is relevant for components, so that the leakage currentproblem is minimized. This also allows any secondary defects formed fromthe oxygen precipitates, such as for example dislocations, to reach thefront side 1, which is relevant for the components. Therefore, on thefront side of the semiconductor wafer and in the layer beneath it thereare no defects which can have an adverse affect on the integratedcircuits.

[0043] Accordingly, the object of the invention is also achieved bymeans of a process for producing a semiconductor wafer with an unevendistribution of crystal lattice defects by means of a heat treatment ata temperature of from 800° C. to 1300° C. The front side of thesemiconductor wafer is exposed to a process gas (gas₁) and the back sideis exposed to a process gas (gas₂) during the heat treatment, with theproviso that gas₁ is not the same as gas₂, but is different from gas₂.

[0044] The heat treatment carried out on the semiconductor waferpreferably takes place in a lamp furnace with two reactor chambers.Furnaces of this nature are known, for example, from EP 0,675,524 A1 orEP 0,476,480 B1. Lamp furnaces also ensure rapid heating and cooling ofthe semiconductor wafer. The profile of the crystal lattice defects isfrozen in particular by suitably rapid cooling. To achieve this, thecooling rates following the heat treatment of the semiconductor waferare preferably between 1 and 300° C./s, particularly preferably between100° C./s and 250° C./s, and in particular between 75° C./s and 200°C./s in the temperature range from 1300° C. to 800° C.

[0045] The concentration and depth of the defects are controlled bymeans of the cooling rates, temperature, duration of the heat treatmentand the process gases used. For example, if the temperature increases,the concentration of defects rises, while a lowering of the cooling rateincreases the PFZ width.

[0046] In the process for producing a semiconductor wafer with an unevendistribution of crystal lattice defects, in which the defectconcentration exhibits a first maximum (max₁) in the central region 7and a second maximum (max₂) in the bottom layer 4, the front side 1 ofthe semiconductor wafer is exposed to a, preferably inert, process gas(gas₁). This gas is preferably selected from a group of gases consistingof nitrogen, hydrogen and the inert gases, as well as any desiredmixtures and any desired inert chemical compounds of the abovementionedgases. If appropriate, traces of oxygen may form up to 10% by volume.

[0047] The back side 2 of the semiconductor wafer is exposed to a,preferably nitriding, process gas (gas₂), which preferably comprisesnitrogen or nitrogen compounds, such as for example ammonia, and, ifappropriate, one or more inert carrier gases.

[0048] In the process for producing a semiconductor wafer with an unevendistribution of crystal lattice defects, in which the defectconcentration exhibits a maximum (max₁) in the upper inner layer 5, thefront side 1 of the semiconductor wafer is exposed to a, preferablyinert, process gas (gas₁), which is preferably selected from a group ofgases consisting of nitrogen, hydrogen and the inert gases, as well asany desired mixtures and any desired inert chemical compounds of theabovementioned gases. If appropriate, traces of oxygen may form up to10% by volume.

[0049] The back side 2 of the semiconductor wafer is exposed to a,preferably oxidizing, process gas (gas₂), which preferably comprisesoxygen or oxygen compounds, such as for example water, and, ifappropriate, one or more inert carrier gases.

[0050] The semiconductor wafer with an uneven distribution of crystallattice defects is converted into a semiconductor wafer with an unevendistribution of nucleation centers for oxygen precipitates by asubsequent heat treatment at temperatures of preferably 300 to 800° C.

[0051] Accordingly, the object of the invention is also achieved bymeans of a process for producing a semiconductor wafer with an unevendistribution of nucleation centers for oxygen precipitates, wherein asemiconductor wafer with an uneven distribution of crystal latticedefects is subjected to a heat treatment at a temperature of between 300and 800° C.

[0052] The duration of the heat treatment is preferably between 1 and360 min, particularly preferably between 30 and 240 min, and inparticular between 60 and 180 min. The heat treatment is preferablycarried out in a process gas atmosphere. The process gas is preferablyselected from a group of gases consisting of oxygen, nitrogen, hydrogenand the inert gases, as well as any desired mixtures and any desiredchemical compounds of the abovementioned gases.

[0053] According to the invention, a profile of nucleation centers foroxygen precipitates which is distinguished by a concentration firstmaximum (max₁) in the central region 7 and a concentration second firstmaximum (max₂) in the bottom layer 4 is generated in the semiconductorwafer.

[0054] According to the invention, a profile of nucleation centers foroxygen precipitates which is distinguished by a concentration maximum(max₁) in the upper inner layer 5 is also generated in the semiconductorwafer.

[0055] By means of profiles of this nature, a precipitate-free layerwith a thickness of from 1 to 100 μm is generated on the front side 1during a subsequent heat treatment process, without outdiffusion ofoxygen being required. The concentration of the oxygen precipitates inthe layers 5 and 6 is preferably set by means of the conditions duringthe formation of the nucleation centers, in particular the temperatureand the time and the duration of the heat treatment in a temperaturerange of preferably from 800° C. to 1300° C.

[0056] Accordingly, while a few embodiments of the present inventionhave been shown and described, it is to be understood that many changesand modifications may be made thereunto without departing from thespirit and scope of the invention as defined in the appended claims.

What is claimed is:
 1. A semiconductor wafer comprising a front side(1), a back side (2), a top layer (3), a bottom layer (4), an upperinner layer (5) lying below the top layer (3), a lower inner layer (6)lying above the bottom layer (4), a central region (7) between layers(5) and (6) and an uneven distribution of crystal lattice defects,wherein the concentration of the defects exhibits a first maximum (max₁)in the central region (7) and a second maximum (max₂) in the bottomlayer (4).
 2. The semiconductor wafer as claimed in claim 1, wherein theconcentration of the defects increases from the front side (1) towardthe central region (7), up to the first maximum (max₁), and theconcentration of defects increases toward the bottom layer (4), up tothe second maximum (max₂).
 3. The semiconductor wafer as claimed claim1, wherein the crystal lattice defects are vacancies.
 4. A semiconductorwafer comprising a front side (1), a back side (2), a top layer (3), abottom layer (4), an upper inner layer (5) lying below the top layer(3), a lower inner layer (6) lying above the bottom layer (4), a centralregion (7) between layers (5) and (6); and an uneven distribution ofcrystal lattice defects, wherein the concentration of the defectsexhibits a maximum (max₁) in the upper inner layer (5).
 5. Thesemiconductor wafer as claimed in claim 4, wherein the concentration ofthe defects increases from the front side (1) of the semiconductor wafertoward the upper inner layer (5).
 6. The semiconductor as claimed inclaim 4, wherein the crystal lattice defects are vacancies.
 7. A processfor producing a semiconductor wafer comprising heat treating asemiconductor wafer at a temperature of from 800° C. to 1300° C.,wherein the front side of the semiconductor wafer is exposed to aprocess gas (gas₁) and the back side is exposed to a process gas (gas₂)during the heat treating, with the proviso that gas₁ is different fromthe gas₂; and wherein said wafer comprises a front side (1), a back side(2), a top layer (3), a bottom layer (4), an upper inner layer (5) lyingbelow the top layer (3), a lower inner layer (6) lying above the bottomlayer (4), a central region (7) between layers (5) and (6) and an unevendistribution of crystal lattice defects, wherein the concentration ofthe defects exhibits a first maximum (max₁) in the central region (7)and a second maximum (max₂) in the bottom layer (4).
 8. The process asclaimed in claim 7, wherein, after the heat treating, cooling the waferat rates of between 1 and 300° C./sec in the temperature range from1300° C. to 800° C.
 9. A semiconductor wafer comprising a front side(1), a back side (2), a top layer (3), a bottom layer (4), an upperinner layer (5) lying below the top layer (3), a lower inner layer (6)lying above the bottom layer (4), a central region (7) between layers(5) and (6) and an uneven distribution of nucleation centers for oxygenprecipitates, wherein the concentration of the nucleation centersexhibits a first maximum (max₁) in the central region (7) and a secondmaximum (max₂) in the bottom layer (4); and the concentration of thenucleation centers on the front side (1) and in the top layer (3) is solow that, in a subsequent heat treatment without outdiffusion of oxygen,a precipitate-free layer with a thickness of from 1 to 100 μm is formedon the front side (1).
 10. A semiconductor wafer comprising a front side(1), a back side (2), a top layer (3), a bottom layer (4), an upperinner layer (5) lying below the top layer (3), a lower inner layer (6)lying above the bottom layer (4), a central region (7) between layers(5) and (6) and an uneven distribution of nucleation centers for oxygenprecipitates, wherein the concentration of the nucleation centersexhibits a maximum (max₁) in the upper inner layer (5); and theconcentration of the nucleation centers on the front side (1), the backside (2), in the top layer (3), the bottom layer (4) and the lower innerlayer (6) is so low that, in a subsequent heat treatment withoutoutdiffusion of oxygen, precipitate-free layers with a thickness of from1 to 100 μm are formed on the front side (1) and the back side (2). 11.A process for producing a semiconductor wafer comprising heat treating asemiconductor wafer with an uneven distribution of crystal latticedefects at a temperature of between 300° C. and 800° C.; and whereinsaid wafer comprises a front side (1), a back side (2), a top layer (3),a bottom layer (4), an upper inner layer (5) lying below the top layer(3), a lower inner layer (6) lying above the bottom layer (4), a centralregion (7) between layers (5) and (6) and an uneven distribution ofnucleation centers for oxygen precipitates, wherein the concentration ofthe nucleation centers exhibits a first maximum (max₁) in the centralregion (7) and a second maximum (max₂) in the bottom layer (4); and theconcentration of the nucleation centers on the front side (1) and in thetop layer (3) is so low that, in a subsequent heat treatment withoutoutdiffusion of oxygen, a precipitate-free layer with a thickness offrom 1 to 100 μm is formed on the front side (1).
 12. The process asclaimed in claim 11, wherein the heat treating lasts for between 15 and360 min.
 13. A process for producing a semiconductor wafer comprisingheat treating a semiconductor wafer with an uneven distribution ofcrystal lattice defects at a temperature of between 300° C. and 800° C.;and wherein said wafer comprises a front side (1), a back side (2), atop layer (3), a bottom layer (4), an upper inner layer (5) lying belowthe top layer (3), a lower inner layer (6) lying above the bottom layer(4), a central region (7) between layers (5) and (6) and an unevendistribution of nucleation centers for oxygen precipitates, wherein theconcentration of the nucleation centers exhibits a maximum (max₁) in theupper inner layer (5); and the concentration of the nucleation centerson the front side (1), the back side (2), in the top layer (3), thebottom layer (4) and the lower inner layer (6) is so low that, in asubsequent heat treatment without outdiffusion of oxygen,precipitate-free layers with a thickness of from 1 to 100 μm are formedon the front side (1) and the back side (2).